1. Field of the Invention
This invention relates to an improved full-function brake valve for use in fluid braking systems used on the semi-trailers of highway tractor-trailer vehicles.
2. Discussion of the Prior Art
To meet all of the emergency and service requirements for highway tractor-trailer combinations, while meeting all of the pertinent governmental regulations, including Federal Regulation FMVSS 121 (Docket 90-3, Notice 2), the trucking industry has resorted to a number of trailer brake valve arrangements which have led to complexity, high costs, installation and maintenance difficulties and related problems.
FMVSS 121 is an important safety standard that applies to highway tractor-trailer vehicles; it requires that the spring brakes, which are spring loaded to engage the brake drums when the braking system air pressure drops below the operating pressure necessary to operate the service brakes, become engaged within certain specified time periods. This provides emergency braking when the service brakes become inoperative.
In a typical semi-trailer braking system, a spring brake control valve may be employed to supply pressurized air to the spring brake chambers to release the spring brakes, and also to exhaust air from the spring brake chambers when the spring brakes are to be again applied. The brake system, however, must be designed so that the spring brakes are not released prematurely, that is, prior to pressurization of the reservoir for operating the service brakes. Safety considerations dictate that the service brake function be enabled by such reservoir pressurization prior to release of the spring brakes. However, it is also desirable that the braking system have a reasonably short period to become effective. Otherwise the user may be tempted to defeat the safety system to avoid long delays in start-up of a tractor-trailer rig.
Illustrative of the prior art is Gross et al. U.S. Pat. No. 4,915,456 and patents cited therein, i.e., Ury U.S. Pat. Nos. 4,042,281, 4,080,004 and 4,163,585; Tannin U.S. Pat. No. 4,182,535; and Carton et al. U.S. Pat. Re. 30,205 (Reissue of U.S. Pat. No. 3,922,064). Still other illustrative prior art includes Fannin U.S. Pat. Nos. 4,472,001 and Campanini 4,593,954 and the patents cited therein. None of these patents disclose or suggest a full-function valve having the features of safety provided by the present invention.
Several prior applications have been filed by Applicants herein, including U.S. application Ser. No. 07/939,141, filed Sep. 2, 1992 in the names of Moody and Koelzer, entitled "Full-Function Valve For Heavy Duty Semi-Trailer Brake Systems", and now issued as U.S. Pat. No. No. 5,236,250, and U.S. application Ser. No. 08/013,588, filed Feb. 4, 1993, in the name of Wallestad, the disclosures of which are hereby incorporated by reference.